The invention relates generally to a power generator, and in particular to reduction of keybar voltages in a power generator.
In order to improve generator efficiency and reduce generator size, generator manufacturers are constantly endeavoring to improve the thermal performance of the generator. For example, a prior art design of a high power electrical generator 100 is illustrated in FIGS. 1 and 2. FIG. 1 is an end view of a cross-section of generator 100 from an isometric perspective. FIG. 2 is a cut-away view of generator 100 along axis 2xe2x80x942. As shown in FIGS. 1 and 2, electrical generator 100 includes a substantially cylindrical stator 102 housing a substantially cylindrical rotor 110. Power generator 100 further includes multiple axially oriented keybars 118 that are circumferentially distributed around an outer surface of the stator 102. Each keybar 118 is mechanically coupled to the outer surface of stator 102. Each keybar 118 is further mechanically coupled at each of a proximal end and a distal end to one of multiple flanges 204. The multiple keybars 118, together with the multiple flanges 204, form a keybar cage around stator 102.
An inner surface of stator 102 includes multiple stator slots 106 that are circumferentially distributed around an inner surface of stator 102. Each stator slot 106 is radially oriented and longitudinally extends approximately a full length of stator 102. Each stator slot 106 receives an electrically conductive stator winding (not shown).
Rotor 110 is rotatably disposed inside of stator 102. An outer surface of rotor 110 includes multiple rotor slots 114 that are circumferentially distributed around the outer surface of rotor 110. Each rotor slot 114 is radially oriented and longitudinally extends approximately a full length of rotor 110. An air gap exists between stator 102 and rotor 110 and allows for a peripheral rotation of rotor 110 about axis 130.
Each rotor slot 114 receives an electrically conductive rotor winding (not shown). Each rotor winding typically extends from a proximal end of rotor 110 to a distal end of the rotor in a first rotor slot 114, and then returns from the distal end to the proximal end in a second rotor slot 114, thereby forming a loop around a portion of the rotor. When a direct current (DC) voltage differential is applied across a rotor winding at the proximal end of rotor 110, an electrical DC current is induced in the winding. Similar to the rotor windings, each stator winding typically extends from a proximal end of stator 102 to a distal end of the stator in a first stator slot 106, and then returns from the distal end of the stator to the proximal of the stator in a second stator slot 106, thereby forming a stator winding loop.
FIG. 3 is a partial perspective of generator of 100 and illustrates a typical technique of constructing a stator core 104. As shown in FIG. 3, stator core 104 includes multiple ring-shaped laminations 402 that are stacked one on top of another in order to build up the core. One design of stator core 104 further includes subdividing each lamination 302 into multiple lamination segments 304. A radially outer surface of each lamination segment 304 includes at least one slot 120 (not shown in FIG. 3) that aligns with one of the multiple keybars 118. Each keybar in turn includes an outer side 124 and an inner, or locking, side 122 that mechanically mates with one of the multiple slots 120. Stator core 104 is then constructed by sliding each lamination segment 304, via one of the multiple slots 120, into the keybar cage formed by the multiple keybars 118. The coupling of each slot of the multiple slots 120 of a lamination segment 304 with a locking side 122 of a keybar 118 affixes each lamination segment in position in stator 102.
A rotation of rotor 110 inside of stator 102 when a DC current is flowing in the multiple windings of rotor 110 induces electromagnetic fields in, and a passage of magnetic flux through, stator 102. A portion of the magnetic flux passes completely through stator 102 and spills outside of the outer surface of stator 102, coupling into each of the multiple keybars 118. The coupling of magnetic flux into each of multiple keybars 118 can induce keybar voltages and keybar currents in each keybar. One possible result is a development of a voltage differential between keybar voltages produced in each of two different keybars 118. When adjacent keybars 118 are coupled to adjacent lamination segments, a voltage differential between the adjacent keybars 118 may also appear across the adjacent lamination segments. The voltage differential between adjacent lamination segments can cause arcing between the two segments, overheating in the stator core 104, and reduced generator performance.
Furthermore, the keybar currents induced in each keybar 118 flow from the keybar 118 to a flange 204 coupled to the keybar. A mechanical joint by which a keybar 118 is coupled to a flange 204 can be a poor electrical conductor that provides a high resistance path for the current. As a result, the joint can be a source of undesirable energy dissipation and heat generation in power generator 100, and is also a potential source of arcing and pitting in the power generator. Furthermore, a flow of keybar current in a magnetically and electrically resistive flange 204 results in undesirable energy and heat dissipation in the flange. To avoid overheating the joint and the flange 204 and potential arcing and pitting, a power generator such as power generator 100 sometimes must be operated at backed off levels of magnetic flux and output voltage, reducing the efficiency and rated power level of the power generator 100.
Therefore, a need exists for a method and apparatus for reducing keybar currents and keybar voltage differentials induced in each of the multiple keybars.
Thus there is a particular need for a method and apparatus that reduces keybar currents and that reduces any voltage differential that may appear between keybars. Briefly, in accordance with an embodiment of the present invention, a keybar shield is provided for insertion adjacent to an outer surface of a stator and that extends approximately an axial length of the stator. The keybar shield reduces the amount of flux coupling into a keybar during operation of a power generator, reducing a keybar voltage and a voltage differential that may appear between keybars. Also, by reducing the amount of flux coupling into a keybar, the keybar shield also reduces keybar currents and flange currents and their associated energy losses.